1. Field of the Invention
This invention generally relates to wireless communications antennas and, more particularly, to a selectable operating frequency antenna formed from a microelectromechanical switch.
2. Description of the Related Art
The size of portable wireless communications devices, such as telephones, continues to shrink, even as more functionality is added. As a result, the designers must increase the performance of components or device subsystems while reducing their size, or placing these components in less desirable locations. One such critical component is the wireless communications antenna. This antenna may be connected to a telephone transceiver, for example, or a global positioning system (GPS) receiver.
Wireless telephones can operate in a number of different frequency bands. In the US, the cellular band (AMPS), at around 850 megahertz (MHz), and the PCS (Personal Communication System) band, at around 1900 MHz, are used. Other frequency bands include the PCN (Personal Communication Network) at approximately 1800 MHz, the GSM system (Groupe Speciale Mobile) at approximately 900 MHz, and the JDC (Japanese Digital Cellular) at approximately 800 and 1500 MHz. Other bands of interest are GPS signals at approximately 1575 MHz and Bluetooth at approximately 2400 MHz.
Typically, better communication results are achieved using a whip antenna. Using a wireless telephone as an example, it is typical to use a combination of a helical and a whip antenna. In the standby mode with the whip antenna withdrawn, the wireless device uses the stubby, lower gain helical coil to maintain control channel communications. When a traffic channel is initiated (the phone rings), the user has the option of extending the higher gain whip antenna. Some devices combine the helical and whip antennas. Other devices disconnect the helical antenna when the whip antenna is extended. However, the whip antenna increases the overall form factor of the wireless telephone.
It is known to use a portion of a circuitboard, such as a dc power bus, as an electromagnetic radiator. This solution eliminates the problem of an antenna extending from the chassis body. Printed circuitboard, or microstrip antennas can be formed exclusively for the purpose of electromagnetic communications. These antennas can provide relatively high performance in a small form factor. However, a wireless device that is expected to operate at a plurality of different frequencies may have difficulty housing a corresponding plurality of microstrip antennas. Even if all the microstrip antennas could be housed, the close proximity of the several microstrip antennas may degrade the performance of each antenna.
FIG. 19 is a schematic diagram of a microelectromechanical switch (MEMS) (prior art). A MEMS is a semiconductor integrated circuit (IC) with an overlying mechanical layer that operates as a selectable connectable switch. That is, the underlying solid-state layer creates a field that can cause an overlying conductive material to move, permitting the conductive material to act as miniature single-pull single-throw switch. MEMS concepts were developed in labs in the 1980's and are just now beginning to be fabricated as practical products. As a result, the particular specifications and features of a MEMS are still under development. MEMS technology offers the possibility of extremely low loss switches miniature switches.
In communications applications, switches are often designed with semiconductor elements such as transistors or pin diodes. At microwave frequencies, however, these devices suffer from several shortcomings. PIN diodes and transistors typically have an insertion loss greater than 1 dB, which is the loss across the switch when the switch is closed. Transistors operating at microwave frequencies tend to have an isolation value of under 20 dB. This allows a signal to “bleed” across the switch even when the switch is open. PIN diodes and transistors have a limited frequency response and typically only respond to frequencies under 20 GHz. In addition, the insertion losses and isolation values for these switches varies depending on the frequency of the signal passing through the switches. These characteristics make semiconductor transistors and pin diodes a poor choice for switches in microwave applications.
As noted in U.S. Pat. No. 6,440,767 (Loo et al.), a microwave MEMS can be made utilizing an armature design. One end of a metal armature is affixed to an output line, and the other end of the armature rests above an input line. The armature is electrically isolated from the input line when the switch is in an open position. When a voltage is applied to an electrode below the armature, the armature is pulled downward and contacts the input line. This creates a conducting path between the input line and the output line through the metal armature. This switch provides only a single-pole, single-throw (SPST) function, that is, the switch is either open or closed.
A SPST MEMS switch can be formed from a multiple-layer armature with a suspended biasing electrode and a conducting transmission line affixed to the structural layer of the armature. A conducting dimple is connected to the conducting line to provide a reliable region of contact for the switch. The switch is fabricated using silicon nitride as the armature structural layer and silicon dioxide as a sacrificial layer supporting the armature during fabrication.
A MEMS switch suitable for RF or microwave applications typically can have a very low insertion loss (less than 0.2 dB at 45 GHz) and a high isolation when open (greater than 30 dB) over a large bandwidth, as compared to semiconductor transistors and pin diodes. These characteristics give the MEMS switch the potential to not only replace traditional narrow-bandwidth PIN diodes and transistor switches in microwave circuits, but to create a whole new class of high performance and compact microwave switch circuits. RF signals often must be switched between two destinations, such as when switching an RF signal between a first antenna array and a second antenna array. Switches that support this configuration are classified as single-pole, double-throw (SPDT) switches.
It would be advantageous if a single wireless communications telephone antenna could be made to operate at a plurality of frequencies.
It would be advantageous if MEMS could be used as part of a microstrip antenna to modify the length of the radiator.